The present invention relates to subscriber telephone sets.
Some sets are provided with amplified loud speaker reception for hearing the correspondant without holding the hand set near the ear.
This loudspeaker consumes considerable energy since it must provide a sound intensity level much higher than that of the loudspeaker of the hand set. Now, the power supply for the subscriber set is wholely supplied by the telephone line itself.
In the case where the power supply for the line is normal (for example during conversation), the energy received is sufficient for supplying the different functions of the set with power, including the supply for an amplifier coupled to the amplified reception loudspeaker. By way of example, a normal line power supply corresponds to a received current of the order of 15 to 150 milliamps at 4 volts.
But in the case where the supply for the line is no longer normal and becomes insufficient, for example if it drops below 10 milliamps, the amplifier of the amplified reception loudspeaker can no longer correctly play its role. In particular, it completely distorts the signals it receives. In general, the amplified reception loudspeaker is only required in cases where the power supply of the line is normal. But in some exceptional cases it may be needed even if the power supplied by the line is abnormally low.
An example of this abnormal situation is the following: some modern telephone sets are provided for allowing line connection without lifting the hand set; thus a number may be dialled without removing the hand set. It may then happen that the user is connected to the line and that, for one reason or another, no complete number is dialled. The telephone exchange is designed to wait until a call number has been dialled when the subscriber has been connected to the line; but if it receives no numbering signal at the end of a certain time, it emits a particular dialling tone and reduces the power supply of the line to a low value. The particular dialling tone is received by the set and is transmitted to the amplifier of the amplified reception loudspeaker. That is necessary for, in this type of set with line connection without the hand set being lifted, there must be absolutely a possibility of knowing whether the line is connected to the telephone exchange without any numbering signal having been sent to the exchange. Unfortunately, in this case, the power supply of the line is so reduced that the amplifier of the amplified reception loudspeaker transmits an extremely distorted signal which does not in practice allow the particular dialling tone set to be recognized or in any case which deforms it in an unacceptable way.
Other cases of distortion exist: in particular, when the signal to be amplified has too great an amplitude and requires for correct amplification power greater than that which is available on the line (even when the line is normally supplied). In addition, in this case, the reservoir capacitor which is often provided for the power supply of an amplified reception loudspeaker risks discharging; this leads to signal distortion which lasts until the capacitor is recharged even if the signal to be amplified has mean time resumed a normal amplitude level.
This is why the present invention proposes modifying the construction of subscriber telephone sets so as to overcome this type of fault.